Paypal  (definitely the easiest and most commonly used)
You can run your business with Paypal for online payments. Your customers can pay via visa debit, credit card or bank transfer making it a popular option.
Canada:
https://www.paypal.com/ca/webapps/mpp/merchant
US:
https://www.paypal.com/webapps/mpp/merchant

Square
Acts as a merchant & gateway.  Super easy to use and integrate.   Allows a seamless payment solution, and integrates with your shopping cart to accept direct credit card payments.  

Canada:
https://squareup.com/ca
US:
https://squareup.com

​Stripe
A bit harder to set up, but works the same as Square.

Canada:
https://stripe.com/ca
US:
https://stripe.com/us